<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>计算圆的面积</title>
</head>
<body>
  <script>
  
  let radius = prompt("请输入圆的半径：")
  radius = Number(radius)
  let area =Math.PI*radius*radius
  document.write("圆的面积为："+area)

 </script> 
</body>
</html>